Evidence is presented for an ester bond linkage of the N-terminal pyruvate containing peptide reductase to the remainder of the protein. The evidence is obtained from experiments in which proline reductase is subjected to one of the following treatments: (1) LiBH4 reduction, (2) short term hydrolysis with 0.1 N NaOH, and (3) 25 percent NH2OH hydrolysis. The same cleavage site on proline reductase is involved in the three different conditions. The iron protein, an electron carrier of the proline reductase complex, is a glycoprotein.